Clock resynchronization circuit and method

ABSTRACT

A control circuit receives a first clock signal at a first frequency, a frequency division signal specifying a divisor number, and a second clock signal at a second frequency (higher than the first frequency). The control circuit includes a phase control block that defines non-overlapping portions of a pulse of the second clock to include center, left and right portions. A determination is then made as to whether an edge of the first clock is located within the center portion. In response to such a determination, a number of periods of the second clock signal which occur within one or more periods of the first clock signal is compared to a number derived from the divisor number to generate a frequency selection signal indicative of that comparison. A controlled oscillator circuit generates the second clock signal at the second frequency, wherein the second frequency is specified by the frequency selection signal. To the extent the edge of the first clock is located within either the left or right portions, phase adjustment is made to move the edge towards the center portion.

TECHNICAL FIELD

The present invention relates to systems and methods for clock recovery.

BACKGROUND

Reference is made to FIG. 1 which illustrates a block diagram of asimplified communications system 10. The system 10 includes atransmitter (TX) 12 and a receiver (RX) 14. The transmitter 12 andreceiver 14 are coupled through an interface 16. The interface 16comprises a data bus connection 18 adapted to support parallelcommunication of a plurality of data bits (for example, eight bits ofDATA[7:0]) from the transmitter 12 to the receiver 14. The interface 16further comprises a clock connection 20 adapted to support communicationof a clock signal (for example, on a single line, as a clock signalCLK_PAD) from the transmitter 12 to the receiver 14. The transmitter 12and receiver 14 are typically circuits, such as integrated circuits,mounted to a circuit board (for example, a printed circuit board) 22.The circuit board 22 includes a plurality of electrical traces (notexplicitly shown) coupled between the transmitter 12 and receiver 14which provide the data bus connection 18 and clock connection 20.

The receiver 14 uses the received clock signal on the clock connection20 to latch the received data bits on the data bus connection 18. Forexample, the receiver 14 may operate to latch data present on the databus connection 18 at the leading edge of the received clock signal.Receipt of the clock signal is thus critical to proper operation of thereceiver 14.

It is known by those skilled in the art that there is an upper limit tothe frequency of the clock signal which can be transmitted over thecircuit board 22 and received by the receiver 14. This upper limit willthen impose a corresponding upper limit on the rate at which the databits can be transmitted over the circuit board 22 and received by thereceiver 14. For example, the circuit board 22 may not be able tosupport a clock signal CLK_PAD transmission at a frequency in excess of100 MHz, and thus the rate of data bit transmission over the data busconnection 18 may be limited to about 50 MHz. There is a need in the artto address the clock frequency upper limit issue noted above inconnection with supporting the interfacing of a transmitter circuit anda receiver circuit, and more specifically to support DATA transmissionover the data bus connection 18 at rates approaching the upper limit forthe circuit board 22.

SUMMARY

In an embodiment, a clock recovery circuit comprises: a control circuitadapted to receive a first clock signal at a first frequency, afrequency division signal specifying a divisor number, and a secondclock signal at a second frequency, wherein the second frequency ishigher than the first frequency. The control circuit comprises: a phasecontrol block adapted to define non-overlapping portions of a pulse ofthe second clock including a center portion, left portion and rightportion and determine whether an edge of the first clock is locatedwithin the center portion; and a frequency control block responsive tothe determination that the edge of the first clock is located within thecenter portion and adapted to compare a number of periods of the secondclock signal which occur within one or more periods of the first clocksignal to a number derived from the divisor number and generate afrequency selection signal in response thereto. The circuit furthercomprises: a controlled oscillator circuit adapted to generate thesecond clock signal at the second frequency, wherein the secondfrequency is specified by the frequency selection signal.

The phase control block is further adapted to determine whether an edgeof the first clock is located within the left portion and in responsethereto generate a phase adjustment signal. The second frequency of thesecond clock signal generated by the controlled oscillator circuit isspecified by both the frequency selection signal and the phaseadjustment signal. The phase adjustment signal causes the edge of thefirst clock to shift towards the center portion.

The phase control block is further adapted to determine whether an edgeof the first clock is located within the right portion and in responsethereto generate a phase adjustment signal. The second frequency of thesecond clock signal generated by the controlled oscillator circuit isspecified by both the frequency selection signal and the phaseadjustment signal. The phase adjustment signal causes the edge of thefirst clock to shift towards the center portion.

The control circuit is adapted to operate in a calibration mode and alock mode. The calibration mode implements an interleaved actuation ofthe phase control block and frequency control block. The lock modeimplements actuation only of the phase control block.

The control circuit is adapted to operate in a fast calibration mode, afine calibration mode and a lock mode. The fast and fine calibrationmodes each implement interleaved actuation of the phase control blockand frequency control block. The lock mode implements actuation only ofthe phase control block.

Actuation of the frequency control block in the fast calibration modecompares the number of periods of the second clock signal which occurwithin only one period of the first clock signal to the divisor numberitself. The frequency selection signal is generated in response thereto.

Actuation of the frequency control block in the fine calibration modecompares the number of periods of the second clock signal which occurwithin a plurality of N periods of the first clock signal to the numberequal to N times the divisor number. The frequency selection signal isgenerated in response thereto.

The frequency selection signal is generated by the frequency controlblock by execution of a binary search algorithm or anincrement/decrement algorithm, wherein each increment or decrement is asingle step.

The size of each of the center portion, left portion and right portionis either fixed or variable. When variable, the size of the centerportion is wider during frequency selection and narrower during phaseselection.

The frequency control block is further adapted to generate the frequencyselection signal having a fractional value.

In another embodiment, a method is provided for implementing theoperations described above in connection with the circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made by way of example only to the accompanyingdrawings in which:

FIG. 1 is a block diagram of a simplified communications system inaccordance with the prior art;

FIG. 2 is a block diagram of a communications system wherein thereceiver includes a clock recovery circuit;

FIG. 3 is a block diagram of the clock recovery circuit;

FIG. 4 illustrates divisional of the clock signal period into multipleparts;

FIG. 5 is a flow diagram for a frequency control loop implemented byfrequency control functionality;

FIG. 6 is a flow diagram for a phase control loop implemented by phasecontrol functionality;

FIG. 7 illustrates a state diagram for operation of a calibration modeand locked mode;

FIG. 8 is flow diagram illustration calibration mode operation;

FIG. 9 is a block diagram of a logic circuit operable to detect relativeposition between two clock signals;

FIGS. 10A-10D are timing diagrams illustration operation of the logiccircuit of FIG. 9;

FIG. 11 illustrates a state diagram for operation of a fast calibrationmode, fine calibration mode and locked mode;

FIG. 12 is a block diagram of a fractional frequency controlfunctionality; and

FIG. 13 illustrates an alternate circuit for signal generation.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is now made to FIG. 2 which illustrates a block diagram of acommunications system 110. The system 110 includes a transmitter (TX)112 and a receiver (RX) 114. The transmitter 112 and receiver 114 arecoupled through an interface 116. The interface 116 comprises a data busconnection 118 adapted to support parallel communication of a pluralityof data bits (for example, eight bits of DATA[7:0]) from the transmitter112 to the receiver 114. The interface 116 further comprises a clockconnection 120 adapted to support communication of a clock signal (forexample, on a single line, as clock CLK_PAD) from the transmitter 112 tothe receiver 114. The transmitter 112 and receiver 114 are typicallycircuits, such as integrated circuits, mounted to a circuit board (forexample, a printed circuit board) 122. The circuit board 122 includes aplurality of electrical traces (not explicitly shown) coupled betweenthe transmitter 112 and receiver 114 which provide the data busconnection 118 and clock connection 120.

The receiver 114 includes a clock recovery circuit 130. The clockrecovery circuit 130 receives the clock signal CLK_PAD on the clockconnection 120 and generates a clock signal CLK that is used internallywithin the receiver 114. For example, the clock signal CLK is used bythe receiver to latch the received data bits on the data bus connection118. For example, the receiver 114 may operate to latch data present onthe data bus connection 118 at the leading edge of the generated clocksignal CLK. In a preferred embodiment, the frequency of the generatedclock signal CLK is greater than the frequency of the received clocksignal CLK_PAD. More specifically, the frequency of the generated clocksignal CLK is a multiple (for example, an integer multiple such as 6×)of the frequency of the received clock signal CLK_PAD). This allows theclock signal CLK_PAD to have a frequency compatible with the circuitboard 122 (i.e., less than the upper limit for the frequency of signaltransmission over the circuit board 122) while still supportingoperation by the receiver 114 to receive and latch the data bitstransmitted on the data bus connection 118 at frequencies approachingthat upper limit. In a particular example, the clock signal CLK_PAD isconfigured to have a frequency of 20 MHz, while the data bits aretransmitted over the data bus connection 118 at a frequency of 60 MHzand latched by the generated clock signal CLK at a frequency of 120 MHz.In another example, the clock signal CLK_PAD is configured to have afrequency of 20 MHz, while the data bits are transmitted over the databus connection 118 at a frequency of 100 MHz and latched by thegenerated clock signal CLK at a frequency of 200 MHz.

Reference is now made to FIG. 3 which illustrates a block diagram of theclock recovery circuit 130. The clock recovery circuit 130 includes acontrolled oscillator circuit 140. The controlled oscillator circuit 140includes a voltage controlled oscillator 142 that outputs the generatedclock signal CLK. The frequency of the clock signal CLK generated andoutput by the voltage controlled oscillator (VCO) 142 is set as afunction of a control voltage Vcap received at an input of the voltagecontrolled oscillator 142. The control voltage Vcap is the voltageacross a capacitor 144 coupled between the input of the voltagecontrolled oscillator 142 and a reference voltage (for example, ground).The control voltage Vcap is set by charging circuitry 146 coupled to thecapacitor 144. The charging circuitry 146 operates responsive to a setof control signals 148 to source charge to the capacitor 144 and draincharge from the capacitor 144, thus setting the control voltage Vcapoutput from the capacitor 144 as a function of that sourced/drainedcharge.

One of the control signals 148 is a charge signal (CHARGE). The CHARGEsignal controls actuation of a current source within the chargingcircuit 146 to source charge to the capacitor 144. Another one of thecontrol signals 148 is a discharge signal (DISCHARGE). The DISCHARGEsignal controls actuation of a current source within the chargingcircuit 146 to drain charge from the capacitor 144. The CHARGE andDISCHARGE signals, along with their associated current sources, and inabsence of any other sourced/drained charge, set Vcap=Vfr and thusdefine the free-running frequency of the generated clock signal CLK.

A set of control signals 148 comprise frequency selection (FREQ_SEL)signals for the controlled oscillator 140. The FREQ_SEL signals controlactuation of a corresponding plurality of current sources within thecharging circuit 146 to source charge to (and perhaps drain charge from)the capacitor 144. The FREQ_SEL signals, along with their associatedcurrent sources, adjust the value of control voltage Vcap=Vo (differentfrom Vfr) so as to set the operating frequency of the generated clocksignal CLK. A change in the FREQ_SEL signals thus effectuates a changein the control voltage Vcap which produces variation in the frequency ofthe clock signal CLK.

Yet another one of the control signals 148 comprises a phase advancesignal (PHASE_ADVANCE). The PHASE_ADVANCE signal controls actuation of acurrent source within the charging circuit 146 to source charge to thecapacitor 144. More specifically, the PHASE_ADVANCE signal causes itscurrent source to momentarily adjust the value of control voltageVcap>Vo (for example, for one cycle of a CLK period) by sourcing chargeto the capacitor 144 so as to provide a momentary increase in theoperating frequency of the generated clock signal CLK for the purpose ofshifting its phase.

A final one of the control signals 148 comprises a phase pullback signal(PHASE_PULLBACK). The PHASE_PULLBACK signal controls actuation of acurrent source within the charging circuit 146 to drain charge from thecapacitor 144. More specifically, the PHASE_PULLBACK signal causes itscurrent source to momentarily adjust the value of control voltageVcap<Vo (for example, for one cycle of a CLK period) by draining chargefrom the capacitor 144 so as to provide a momentary decrease in theoperating frequency of the generated clock signal CLK for the purpose ofshifting its phase.

The clock recovery circuit 130 further includes a digitalphase-lock-loop (PLL) control circuit 160 which generates the controlsignals 148. The digital PLL control circuit 160 operates responsive toa set of signals 162. One of the signals 162 is the generated clocksignal CLK (or signals derived from or related to the clock signal CLK)output from the controlled oscillator circuit 140. Another of thesignals is the clock signal CLK_PAD supplied by the transmitter 112. Thedigital PLL control circuit 160 further receives a set of frequencydivide signals (FREQ_DIV) as part of the signals 162. The FREQ_DIVsignals provide a numerical divisor whose value specifies a frequencyratio between the clock signal CLK_PAD supplied by the transmitter 112and the generated clock signal CLK output from the controlled oscillatorcircuit 140.

The digital PLL control circuit 160 includes a frequency controlfunctionality 170 and a phase control functionality 172.

The frequency control functionality 170 performs a check of theoperating frequency of the generated clock signal CLK. This checkincludes an operation to count the number of CLK cycles within oneCLK_PAD period. The goal is for the number of CLK cycles within oneCLK_PAD period to equal the value of the numerical divisor provided bythe FREQ_DIV signals. If the counted number does not match the value ofthe numerical divisor, then the frequency of the generated clock signalCLK is incorrect and the frequency control functionality 170 will changethe value of the FREQ_SEL signals output from the digital PLL controlcircuit 160 and applied to the controlled oscillator circuit 140.

In evaluating the generated clock signal CLK, each CLK period is dividedby the digital PLL control 160 into four parts referred to operatingpoints (OP): OP1, OP2, OP3 and OP4 as shown in FIG. 4. Preferably, theseoperating points are non-overlapping. The part OP1 is generally thecenter portion of the positive pulse (or first half) of the CLK period.The part OP2 is generally a portion of the positive pulse of the CLKperiod between the part OP1 and a trailing edge of the positive pulse.The part OP3 is generally a portion of the positive pulse of the CLKperiod between a leading edge of the positive pulse and the part OP1.The part OP4 is the negative pulse (or second half) of the CLK period.

FIG. 4 further illustrates an exemplary timing relationship between thegenerated clock signal CLK and the clock signal CLK_PAD. The operationto change the value of the FREQ_SEL signals output from the digital PLLcontrol circuit 160 is performed by the frequency control functionality170 only when the leading (or trailing) edges of the clock signalCLK_PAD fall within the part OP1 of the generated clock signal CLK. Toensure accuracy, the operation to count the number of CLK cycles withinone CLK_PAD period is performed twice: once with respect to the leading(positive) edges of the generated clock signal CLK; and once withrespect to the trailing (negative) edges of the generated clock signalCLK. Only if the counts are made when both the leading (or trailing)edges of the clock signal CLK_PAD fall within the part OP1 of thegenerated clock signal CLK can it be ensured that the counts areaccurate and can be used to drive any necessary frequency adjustmentthrough a change in the value of the FREQ_SEL signals.

FIG. 5 provides a flow diagram for a frequency control loop 200implemented by the frequency control functionality 170. The loop 200tests at step 202 whether the leading (or trailing) edges of the clocksignal CLK_PAD fall within the part OP1 of the generated clock signalCLK. If not, the process returns 204 to step 202 and checks again. Whenthe test in step 202 is satisfied, the loop 200 implements a frequencycontrol algorithm 206. The algorithm 206 preferably makes use of abinary search to speed up the frequency tracking operation, but thisbinary search is optional and instead a more simple algorithmimplementing an increment/decrement operation could be used. Thealgorithm 206 controls the selection of the frequency of the generatedclock signal CLK by: a) counting the number of CLK cycles within oneCLK_PAD period; b) comparing the count to the value of the numericaldivisor provided by the FREQ_DIV signals; and setting the value of theFREQ_SEL signals in response to that comparison. In this case, settingmay comprise changing or not changing the previously set value of theFREQ_SEL signals as necessary in accordance with the result of the countand comparison. The loop 200 then enters a wait or delay mode 208 beforereturning 210 to step 202 and check again.

The phase control functionality 172 performs a check of the operatingpoint of the generated clock signal CLK. It will be recalled that thepreference is for either the leading edge or trailing edge of the clocksignal CLK_PAD to fall within the part OP1 of the generated clock signalCLK. The check performed by the phase control functionality 172accordingly includes an operation to determine the locations of theleading (or trailing) edges of the clock signal CLK_PAD relative to theoperating points (OP) of the CLK period.

Reference is once again made to FIG. 4. To the extent the rising edge ofthe clock signal CLK_PAD falls within the OP2 part of the CLK period,this is indicative of the phase of the generated clock signal CLK beingearly and a phase pullback operation is instigated by asserting thePHASE_PULLBACK signal to temporarily (for example, for one CLK period)slow down the clock signal CLK and bring its phase back into alignmentwith the OP1 part. Conversely, to the extent the rising edge of theclock signal CLK_PAD falls within the OP3 part of the CLK period, thisis indicative of the phase of the generated clock signal CLK being lateand a phase advance operation is instigated by asserting thePHASE_ADVANCE signal to temporarily (for example, for one CLK period)speed up the clock signal CLK and bring its phase back into alignmentwith the OP1 part. Lastly, to the extent the rising edge of the clocksignal CLK_PAD falls within the OP4 part of the CLK period, this isindicative of the phase of the generated clock signal CLK being out ofphase (or in the wrong phase) and a phase change (advance or pullback)operation is instigated (using the PHASE_ADVANCE signal orPHASE_PULLBACK signal) over an extended period of time (for example,over multiple CLK periods) to change the frequency of the clock signalCLK and bring its phase back into alignment with the OP1 part.

FIG. 6 provides a flow diagram for a phase control loop 250 implementedby the phase control functionality 172. The loop 250 tests at step 252whether the leading (or trailing) edges of the clock signal CLK_PAD fallwithin the part OP1 of the generated clock signal CLK. If so, theprocess returns 254 to step 252 and checks again. It is preferred thateither the leading edge or trailing edge of the clock signal CLK_PADfall within the part OP1, and the remainder of the phase control loop250 concerns the operations taken to bring the leading (or trailing)edges of the clock signal CLK_PAD within the part OP1.

If no in step 252, then the loop 250 tests at step 256 whether theleading (or trailing) edges of the clock signal CLK_PAD fall within thepart OP2 of the generated clock signal CLK. If so, this indicates thatthe phase of the clock is early, and perhaps that the clock signal CLKis running too fast. A phase pullback operation is then performed.

There are two types of phase pullback supported by the loop 250. Atimeout test is performed at step 258 to determine the cumulative amountof time the leading (or trailing) edges of the clock signal CLK_PAD aredetected within the part OP2 without successfully moving to part OP1. Ifthe cumulative time does not exceed a threshold for OP2, then a singlephase pullback operation 260 is instigated by asserting thePHASE_PULLBACK signal to temporarily (for example, for one CLK period)slow down the clock signal CLK and bring its phase back into alignmentwith the OP1 part. Conversely, if the cumulative time exceeds thethreshold for OP2, then a double phase pullback operation 262 isinstigated by asserting the PHASE_PULLBACK signal to temporarily (forexample, for two or more CLK periods) slow down the clock signal CLK andbring its phase back into alignment with the OP1 part. The loop 250 thenreturns 254 to step 252 and checks again.

If no in step 256, then the loop 250 tests at step 264 whether theleading (or trailing) edges of the clock signal CLK_PAD fall within thepart OP3 of the generated clock signal CLK. If so, this indicates thatthe phase of the clock is late, and perhaps that the clock signal CLK isrunning too slow. A phase advance operation is then performed.

There are two types of phase pullback supported by the loop 250. Atimeout test is performed at step 266 to determine the cumulative amountof time the leading (or trailing) edges of the clock signal CLK_PAD aredetected within the part OP3 without successfully moving to part OP1. Ifthe cumulative time does not exceed a threshold for OP3, then a singlephase pullback operation 268 is instigated by asserting thePHASE_PULLBACK signal to temporarily (for example, for one CLK period)slow down the clock signal CLK and bring its phase back into alignmentwith the OP1 part. Conversely, if the cumulative time exceeds thethreshold for OP3, then a double phase pullback operation 270 isinstigated by asserting the PHASE_PULLBACK signal to temporarily (forexample, for two or more CLK periods) slow down the clock signal CLK andbring its phase back into alignment with the OP1 part. The loop 250 thenreturns 254 to step 252 and checks again.

If no in step 264, then the leading (or trailing) edges of the clocksignal CLK_PAD must fall within the part OP4 of the generated clocksignal CLK, and thus the clock signal CLK_PAD is out of phase (or in thewrong phase), and perhaps the clock signal CLK frequency is notcorrectly set. A phase change operation is then performed.

There are two types of phase pullback supported by the loop 250. Atimeout test is performed at step 272 to determine the cumulative amountof time the leading (or trailing) edges of the clock signal CLK_PAD aredetected within the part OP4 without successfully moving to part OP1 (orparts OP2 or OP3). If the cumulative time does not exceed a thresholdfor OP4, then a single phase pullback operation 274 is instigated byasserting the PHASE_PULLBACK signal to temporarily (for example, for oneCLK period) slow down the clock signal CLK and bring its phase back intoalignment with the OP1 part (or parts OP2 or OP3). Conversely, if thecumulative time exceeds the threshold for OP4, then a double phasepullback operation 276 is instigated by asserting the PHASE_PULLBACKsignal to temporarily (for example, for two or more CLK periods) slowdown the clock signal CLK and bring its phase back into alignment withthe OP1 part (or parts OP2 or OP3). The loop 250 then returns 254 tostep 252 and checks again.

Although the phase change associated with the OP4 comparison isdescribed above as using a phase advance operation, it will beunderstood that a phase pullback operation could alternatively be used.

The digital PLL control circuit 160 operates in two modes: a calibrationmode and a locked mode. The calibration mode comprises a controlledinterleaving of the frequency control functionality 170 and the phasecontrol functionality 172. In calibration mode, a frequency binarysearch is preferably used to reduce the number of search stepsassociated with selecting a frequency (but it will be understood that asimple increment/decrement operation could instead be used). Thefrequency is calibrated using both edges of the CLK period so as toreduce the frequency count accuracy to half of the CLK period. Furtheraccuracy can be achieved counting the number of CLK periods within alarger number of CLK_PAD cycles. The calibration mode still further usesthe phase control which implements a phase look-ahead type process so asto minimize the phase jitter. To effectuate this jitter minimization,the division point between OP1 and OP2 and the division point betweenOP1 and OP3 are set, for example, in a fixed relationship, at orslightly less than one-quarter of the CLK period. With this setting, theCLK_PAD jitter must also be at or less than one-quarter of the CLKperiod. In calibration mode, testing as to each of the operating pointsOP1, OP2, OP3 and OP4 is permitted. In locked mode, however, only theoperating points OP1, OP2 and OP3 are evaluated.

Reference is now made to FIG. 7 which illustrates a state diagram foroperation of the calibration mode and locked mode. The calibration modeis entered by a start signal 300 from an idle state 302. Responsive tosignal 300, the calibration mode enters a phase adjustment state 304.This phase adjustment state 304, as described above in connection withthe phase control functionality 172 and implementation of the phasecontrol loop 250, performs a check of the operating point of thegenerated clock signal CLK (relative to the parts OP1, OP2, OP3 and OP4)and initiates phase advance operations or phase pullback operations asneeded. If no phase change is made, the calibration mode moves 306 tothe frequency adjustment state 308. Conversely, to the extent a phasechange is made (by assertion of either the PHASE_ADVANCE signal or thePHASE_PULLBACK signal), a move 310 is made in the calibration mode toenter a wait state 312. This wait state 312 introduces process delayused to avoid chattering after any operational adjustment, such as aphase adjustment, is made. After the delay expires, the calibration modechecks if the signal is in OP1 and then moves 314 to the frequencyadjustment state 308. If not in OP1, the process returns 320 to thephase adjustment state 304. This frequency adjustment state 308, asdescribed above in connection with the frequency control functionality170 and implementation of the frequency control loop 200, performs acheck on the operating frequency of the generated clock signal CLK bycomparing a count of the number of CLK cycles within one CLK PAD periodto the numerical divisor provided by the FREQ_DIV signals and initiatinga frequency change as needed. If no phase change is made, thecalibration mode moves 316 back to the phase adjustment state 304. Tothe extent a frequency change is made (by changing the value of theFREQ_SEL signals), the calibration mode enters 318 the wait state 312.This wait state 312 introduces process delay used to avoid chatteringafter any operational adjustment, such as a frequency adjustment, ismade. After the delay expires, the calibration mode moves 320 back tothe phase adjustment state 304.

It will accordingly be noted that the calibration mode implements aninterleaved phase and frequency adjustment technique. The calibrationmode may bounce back and forth in this interleaved operation between thephase adjustment state 304 and frequency adjustment state 308 for afixed number of times before exiting the calibration mode, oralternatively may bounce back and forth in this interleaved operationbetween the phase adjustment state 304 and frequency adjustment state308 until a certain condition is met concerning phase and frequencysetting (such as, for example, the calibration mode passes consecutivelythrough the phase adjustment state 304 and frequency adjustment state308 without making any phase or frequency change).

Upon exiting 322 the calibration mode, movement is made to the lockedmode with locked state 324. The locked state 324 implements a modifiedversion of phase adjustment, as described above in connection with thephase control functionality 172 and implementation of the phase controlloop 250, by performing a check of the operating point of the generatedclock signal CLK (relative to the parts OP1, OP2 and OP3) and initiatingphase advance operations or phase pullback operations as needed. Asdiscussed above, there is no evaluation made in the locked mode 324 withrespect to the part OP4. To the extent a phase change is needed, thelocked mode 324 supports an assertion of either the PHASE_ADVANCE signalor the PHASE_PULLBACK signal. The phase adjustment operation performedin the locked mode 324 may be viewed as a fine tuning of phase withrespect to the signal CLK at the previously locked frequency.

The controlled interleaving of the frequency control functionality 170and the phase control functionality 172 as performed by the calibrationmode is illustrated in flow diagram format in FIG. 8. The process testsat step 330 whether the leading (or trailing) edges of the clock signalCLK_PAD fall within the part OP1 of the generated clock signal CLK. Ifno, this indicates an out of phase condition and the process enters thephase adjustment state 304 as described above. Following completion of aphase adjustment (advance or pullback), the process enters the waitstate 312 and then returns to step 330. If step 330 indicates that theleading (or trailing) edges of the clock signal CLK_PAD fall within thepart OP1 of the generated clock signal CLK, the process enters thefrequency adjustment state 308 as described above. To the extent afrequency adjustment is performed, the process enters the wait state 312and then returns to step 330. If no frequency adjustment is performed,the process exits 332 to the locked state.

The frequency control algorithm 206 of the frequency control loop 200performs a check on the operating frequency of the generated clocksignal CLK by comparing a count of the number of CLK cycles within oneCLK_PAD period to the numerical divisor provided by the FREQ_DIVsignals. More specifically, the algorithm 206 performs this count withrespect to both the rising and falling edges of the clock signal CLK toproduce two count values (COUNTN and COUNTP). If both COUNTN and COUNTPare less than the FREQ_DIV value, then the VCO is operating too slowly(the clock signal CLK is slow) and the FREQ_SEL value is increased (forexample, by incrementing by one step or executing the binary search).Conversely, if both COUNTN and COUNTP are more than the FREQ_DIV value,then the VCO is operating too quickly (the clock signal CLK is fast) andthe FREQ_SEL value is decreased (for example, by decrementing by onestep or implementing the binary search). Before making another frequencycheck after a frequency change, it is important to again achieve phasealignment by executing the phase control loop. When both COUNTN andCOUNTP equal the FREQ_DIV value, then the VCO is operating at thecorrect frequency and the frequency may be locked and the frequencytracking process terminated in favor of continued phase observation inthe locked mode.

It will be noted that an initial frequency of the clock signal CLK(during a first pass through the frequency control process) could besignificantly offset from the desired frequency. The process describedabove for implementing a one step increment or decrement of the FREQ_SELvalue may take too long to achieve frequency lock. The frequency controlalgorithm 206 accordingly supports the use of a binary search algorithmwith respect to controlling the selection of the FREQ_SEL value whenCOUNTN and COUNTP are determined to differ from the FREQ_DIV value.Binary search algorithms are well known to those skilled in the art. Asan example of the search implementation, the magnitude of the incrementor decrement of the FREQ_SEL value is made as a power of 2 thatdecreases with each successive frequency adjustment. So, a firstadjustment may comprise an increment or decrement of the FREQ_SEL valueby 8 (2³), while a second adjustment may comprise an increment ordecrement of the FREQ_SEL value by 4 (2²), then a third adjustment maycomprise an increment or decrement of the FREQ_SEL value by 2 (2¹), anda final adjustment may comprise an increment or decrement of theFREQ_SEL value by 1 (2⁰).

Reference is now made to FIG. 9 which illustrates a logic circuit 378that will logically combine the clock signal CLK and clock signalCLK_PAD to determine whether the leading (or trailing) edges of theclock signal CLK_PAD fall within parts OP1, OP2, OP3 and OP4 of thegenerated clock signal CLK. In this implementation, the clock signal CLKis obtained from the VCO 142 after processing through a first delay 380.Thus, the output of the VCO 142 is a signal CLKM. The clock signal CLKis processed through a second delay 382 to produce a signal CLKP. Thus,signals CLKM and CLKP have the same frequency, but differing phases(with CLKM ahead of CLK and CLKP behind CLK). It will be understood thatother circuitry than that illustrated in FIG. 9 could be used togenerate the signals CLKM and CLKP having the described phaserelationship with respect to the clock signal CLK.

A first D-type latch 384 receives the clock signal CLK_PAD at its data(D) input and an inverted 386 clock signal CLK at its clock (C) input.The first D-type latch 384 outputs a signal CLK_PADN which is the clocksignal CLK_PAD latched by the negative edge of the clock signal CLK.

A second D-type latch 388 receives the clock signal CLK_PAD at its data(D) input and the clock signal CLK at its clock (C) input. The secondD-type latch 388 outputs a signal CLK_PADP which is the clock signalCLK_PAD latched by the positive edge of the clock signal CLK.

A third D-type latch 390 receives the clock signal CLK_PAD at its data(D) input and an inverted 392 signal CLKM at its clock (C) input. Thethird D-type latch 390 outputs a signal CLKM_PADN which is the clocksignal CLK_PAD latched by the negative edge of the signal CLKM.

A fourth D-type latch 392 receives the clock signal CLK_PAD at its data(D) input and the signal CLKP at its clock (C) input. The fourth D-typelatch 392 outputs a signal CLKP_PADP which is the clock signal CLK_PADlatched by the positive edge of the clock signal CLKP.

A first NAND gate 394 receives the clock signal CLK_PAD at its firstinput, an inverted 396 signal CLKM_PADN at its second input, and aninverted 398 signal CLK_PADN at its third input. The first NAND gate 394outputs a signal OP2 which is indicative of the leading (or trailing)edges of the clock signal CLK_PAD falling within part OP2 of thegenerated clock signal CLK. A pulse generator 400 outputs a pulsedsignal DETECT OP2 in response to the signal OP2.

A second NAND gate 402 receives the clock signal CLK_PAD at its firstinput, an inverted 404 signal CLK_PADP at its second input, and aninverted 406 signal CLKP_PADP at its third input. The second NAND gate402 outputs a signal OP3 which is indicative of the leading (ortrailing) edges of the clock signal CLK_PAD falling within part OP3 ofthe generated clock signal CLK. A pulse generator 408 outputs a pulsedsignal DETECT OP3 in response to the signal OP3.

A third NAND gate 410 receives the clock signal CLK_PAD at its firstinput, an inverted 398 signal CLK_PADN at its second input, and aninverted 404 signal CLK_PADP at its third input. The third NAND gate 410outputs a signal OP4 which is indicative of the leading (or trailing)edges of the clock signal CLK_PAD falling within part OP4 of thegenerated clock signal CLK. A pulse generator 412 outputs a pulsedsignal DETECT OP4 in response to the signal OP4.

Operation of the logic circuit 378 to make a detection of the leading(or trailing) edges of the clock signal CLK_PAD falling within part OP1of the generated clock signal CLK is shown in FIG. 10A. It will be notedthat detection within part OP1 is indicated when there is no change inthe signal DETECT OP2, signal DETECT OP3 or signal DETECT OP4. Thisindicates phase alignment as discussed above, and the OP1 detection canbe used to trigger movement from the phase adjustment state 304 to thefrequency adjustment state 308 during calibration mode.

Operation of the logic circuit 378 to make a detection of the leading(or trailing) edges of the clock signal CLK_PAD falling within part OP2of the generated clock signal CLK is shown in FIG. 10B. Output of thepulsed signal DETECT OP2 indicates detection within part OP2. Thisindicates that the phase of the clock signal CLK is early and thatperhaps the clock frequency is too fast. The OP2 detection can be usedto trigger the PHASE_PULLBACK signal to effectuate a phase adjustment ineither the phase adjustment state 308 during calibration mode or thelocked state 324 in the locked mode. For example, the PHASE_PULLBACKsignal may be activated for the duration of one clock period of theclock signal CLK to momentarily decrease the frequency of the clocksignal CLK and move the leading (or trailing) edges of the clock signalCLK_PAD towards, and preferably within, the part OP1.

Operation of the logic circuit 378 to make a detection of the leading(or trailing) edges of the clock signal CLK_PAD falling within part OP3of the generated clock signal CLK is shown in FIG. 10C. Output of thepulsed signal DETECT OP3 indicates detection within part OP3. The OP3detection can be used to trigger the PHASE_ADVANCE signal to effectuatea phase adjustment in either the phase adjustment state 308 duringcalibration mode or the locked state 324 in the locked mode. Forexample, the PHASE_ADVANCE signal may be activated for the duration ofone clock period of the clock signal CLK to momentarily increase thefrequency of the clock signal CLK and move the leading (or trailing)edges of the clock signal CLK_PAD towards, and preferably within, thepart OP1.

Operation of the logic circuit 378 to make a detection of the leading(or trailing) edges of the clock signal CLK_PAD falling within part OP4of the generated clock signal CLK is shown in FIG. 10D. Output of thepulsed signal DETECT OP4 indicates detection within part OP4. The OP4detection can be used to trigger the PHASE_ADVANCE signal to effectuatea phase adjustment in the phase adjustment state 308. There is no needto evaluate the detection of the part OP4 in while in the locked state324 of the locked mode. For example, the PHASE_ADVANCE signal may beactivated for the duration of one or more clock periods of the clocksignal CLK to momentarily increase the frequency of the clock signal CLKand move the leading (or trailing) edges of the clock signal CLK_PADtowards, and preferably within, the part OP1.

Again, although the part OP4 action described above involves a phaseadjustment using the PHASE_ADVANCE signal, it will be understood thatalternatively the phase adjustment could use the PHASE_PULLBACK signal.

In an alternative embodiment, the digital PLL control circuit 160operates in three modes: a fast calibration mode, a fine calibrationmode and a locked mode. Each of the two calibration modes comprises acontrolled interleaving of the frequency control functionality 170 andthe phase control functionality 172. In the fast calibration mode, afrequency binary search is used to reduce the number of search stepsassociated with a course selection of the clock frequency. In the finecalibration mode, a single step search is used to fine tune the clockfrequency. In both calibration modes, the frequency is calibrated usingboth edges of the CLK period so as to reduce the frequency countaccuracy to half of the CLK period. In the fast calibration mode,account is made of the number of CLK periods within a single CLK_PADcycle. In the fine calibration mode, however, further accuracy isachieved by counting the number of CLK periods within a larger number ofCLK_PAD cycles. Each calibration mode still further uses the phasecontrol which implements a phase look-ahead type process so as tominimize the phase jitter. To effectuate this jitter minimization, thedivision point between OP1 and OP2 and the division point between OP1and OP3 are set at or slightly less than one-quarter of the CLK period.With this setting, the CLK_PAD jitter must also be at or less thanone-quarter of the CLK period. In either calibration mode, testing as toeach of the operating points OP1, OP2, OP3 and OP4 is permitted. Inlocked mode, however, only the operating points OP1, OP2 and OP3 areevaluated.

Reference is now made to FIG. 11 which illustrates a state diagram foroperation of the fast calibration mode, fine calibration mode and lockedmode. The fast calibration mode is entered by a start signal 300 from anidle state 302. Responsive to signal 300, the fast calibration modeenters a phase adjustment state 304. This phase adjustment state 304, asdescribed above in connection with the phase control functionality 172and implementation of the phase control loop 250, performs a check ofthe operating point of the generated clock signal CLK (relative to theparts OP1, OP2, OP3 and OP4) and initiates phase advance operations orphase pullback operations as needed. If no phase change is made, thefast calibration mode moves 306 to the frequency adjustment state 308.Conversely, to the extent a phase change is made (by assertion of eitherthe PHASE_ADVANCE signal or the PHASE_PULLBACK signal), a move 310 ismade in the calibration mode to enter a wait state 312. This wait state312 introduces process delay used to avoid chattering after anyoperational adjustment, such as a phase adjustment, is made. After thedelay expires, the fast calibration mode moves checks that the signal isin OP1 and then moves 314 to the frequency adjustment state 308. If notin OP1, the process returns 320 to the phase adjust state. Thisfrequency adjustment state 308, as described above in connection withthe frequency control functionality 170 and implementation of thefrequency control loop 200, performs a check on the operating frequencyof the generated clock signal CLK by comparing a count of the number ofCLK cycles within one CLK_PAD period to the numerical divisor providedby the FREQ_DIV signals and initiating a frequency change as needed. Thefrequency change operation utilizes the binary search techniquedescribed above. If no frequency change is made, the calibration modemoves 316 back to the phase adjustment state 304. To the extent afrequency change is made (by changing the value of the FREQ_SELsignals), the calibration mode enters 318 the wait state 312. This waitstate 312 introduces a process delay used to avoid chattering after anyoperational adjustment, such as a frequency adjustment, is made. Afterthe delay expires, the calibration mode moves 320 back to the phaseadjustment state 304.

It will accordingly be noted that the fast calibration mode implementsan interleaved phase and frequency adjustment technique. The fastcalibration mode may bounce back and forth in an interleaved mannerbetween the phase adjustment state 304 and frequency adjustment state308 for a fixed number of times before exiting the fast calibrationmode, or alternatively may bounce back and forth in an interleavedmanner between the phase adjustment state 304 and frequency adjustmentstate 308 until a certain condition is met concerning phase andfrequency setting (such as, for example, the fast calibration modepasses consecutively through the phase adjustment state 304 andfrequency adjustment state 308 without making any phase or frequencychange).

Upon exiting 350 the fast calibration mode, movement is made to the finecalibration mode. The fine calibration mode first enters a phaseadjustment state 354. This phase adjustment state 354, as describedabove in connection with the phase control functionality 172 andimplementation of the phase control loop 250, performs a check of theoperating point of the generated clock signal CLK (relative to the partsOP1, OP2, OP3 and OP4) and initiates phase advance operations or phasepullback operations as needed. If no phase change is made, the finecalibration mode moves 356 to the frequency adjustment state 358.Conversely, to the extent a phase change is made (by assertion of eitherthe PHASE_ADVANCE signal or the PHASE_PULLBACK signal), a move 360 ismade in the calibration mode to enter a wait state 362. This wait state362 introduces process delay used to avoid chattering after anyoperational adjustment, such as a phase adjustment, is made. After thedelay expires, the fine calibration mode check if the signal is in OP1and then moves 364 to the frequency adjustment state 358. If not in OP1,the process returns 370 to the phase adjustment state 354. Thisfrequency adjustment state 358, as described above in connection withthe frequency control functionality 170 and implementation of thefrequency control loop 200, performs a check on the operating frequencyof the generated clock signal CLK by comparing a count of the number ofCLK cycles within several (for example, N, where N>1) CLK_PAD periods tothe numerical divisor provided by the FREQ_DIV signals (multiplied by N)and initiating a frequency change as needed. The frequency changeoperation utilizes a single step adjustment technique. If no frequencychange is made, the fine calibration mode moves 366 back to the phaseadjustment state 354. To the extent a frequency change is made (bychanging the value of the FREQ_SEL signals), the calibration mode enters368 the wait state 362. This wait state 362 introduces a process delayused to avoid chattering after any operational adjustment, such as afrequency adjustment, is made. After the delay expires, the calibrationmode moves 370 back to the phase adjustment state 354.

It will accordingly be noted that the fine calibration mode alsoimplements an interleaved phase and frequency adjustment technique. Thefine calibration mode may bounce back and forth in an interleaved mannerbetween the phase adjustment state 354 and frequency adjustment state358 for a fixed number of times before exiting the fine calibrationmode, or alternatively may bounce back and forth in an interleavedmanner between the phase adjustment state 354 and frequency adjustmentstate 358 until a certain condition is met concerning phase andfrequency setting (such as, for example, the fine calibration modepasses consecutively through the phase adjustment state 354 andfrequency adjustment state 358 without making any phase or frequencychange).

Upon exiting 322 the fine calibration mode, movement is made to thelocked mode with locked state 324. The locked state 324 implements amodified version of phase adjustment, as described above in connectionwith the phase control functionality 172 and implementation of the phasecontrol loop 250, by performing a check of the operating point of thegenerated clock signal CLK (relative to the parts OP1, OP2 and OP3) andinitiating phase advance operations or phase pullback operations asneeded. As discussed above, there is no evaluation made in the lockedmode 324 with respect to the part OP4. To the extent a phase change isneeded, the locked mode 324 supports an assertion of either thePHASE_ADVANCE signal or the PHASE_PULLBACK signal. The phase adjustmentoperation performed in the locked mode 324 may be viewed a fine tuningof phase with respect to the signal CLK at the previously lockedfrequency.

Reference is once again made to FIG. 3. As discussed above, the FREQ_SELsignals are processed by the charging circuit 146 to select a certainvoltage Vcap on the capacitor 144, with that voltage driving the outputfrequency of the signal CLK from the VCO 142. When making a frequencyadjustment, the frequency control 170 selects the value of the FREQ_SELsignals. A change in the value of the FREQ_SEL signals plus/minus one isthe fine tuning limit exercised over the setting of the output frequencyof the signal CLK. Situations may arise, however, where the plus/minusone fine tuning limit is not satisfactory. A need would exist to providesome additional fractional control over frequency fine tuning. Such aneed exists, for example, in connection with the operation of the finecalibration mode in FIG. 11.

Reference is now made to FIG. 12 which illustrates a block diagram of afractional frequency control functionality within the frequency control170. A divider circuit 500 receives a signal UP which is pulsed inresponse to a fine increment signal (FINE_INC), and a signal DOWN whichis pulsed in response to a fine decrement (FINE_DEC) signal. TheseFINE_INC and FINE_DEC signals provide for fractional adjustment usingthe FREQ_SEL value. The divider circuit 500 performs a division (divideby P) and outputs an increment signal INC and a decrement signal DEC.The value of P defines a fractional level. For example, if P=4, thenfractional steps of four (with four shift registers) are implemented. Afractional counter 502 receives both the increment signal INC anddecrement signal DEC as well as a LOAD signal (used to load in initialvalue into the fractional counter 502). When the increment signal INC isasserted, a count value within the fractional counter 502 is incrementedby one. When the decrement signal DEC is asserted, a count value withinthe fractional counter 502 is decremented by one. The fractionalfrequency control functionality further includes a shift register 504.The shift register 504 includes a plurality of register locations forstoring count values. In response to assertion of either the signal UPor the signal DOWN, the shift register 504 loads the current count value(CNT_VALUE) from the fractional counter 502 into a first one of theregister locations (CNT_VALUE0) and pushes previously loaded countvalues down to the other register locations (CNT_VALUE1 throughCNT_VALUE3). A multiplexer 506 receives each of the count values fromregister locations CNT_VALUE0 through CNT_VALUE3 and selects one of thecount values for output as the frequency select signal FREQ_SEL. Morespecifically, the multiplexer 506 is clocked by the signal CLK to selectone of the count values from register locations CNT_VALUE0 throughCNT_VALUE3 in a round robin (i.e., cyclical) manner. In this way, thefrequency of the clock CLK, as set by the frequency select signalFREQ_SEL, can be fractionally controlled by the count values fromregister locations CNT_VALUE0 through CNT_VALUE3. To understand how thisworks, assume that register locations CNT_VALUE0 through CNT_VALUE3store the following count values: 10, 10, 10, 11. Over the course offour clock CLK cycles, the multiplexer 506 will assert those countvalues: 10, 10, 10, 11 to the charging circuit 146 as the frequencyselect signal FREQ_SEL so as to control the current sources within thecharging circuit 146 and set the voltage Vcap on the capacitor 144 atthe input of the VCO 142. The capacitor 144 functions to average thevoltages Vcap corresponding to the count values: 10, 10, 10, 11 andproduce a fractional value of 10.25. The output frequency of the clockCLK can thus be fractionally set.

Reference is once again made to FIG. 9 which shows the use of a firstdelay 380 and a second delay 382 in connection with generating thesignals CLKM, CLK and CLKP. In a preferred implementation, those delaysare fixed and thus the relative positions of the division point betweenOP1 and OP2 and the division point between OP1 and OP3 are also fixed.For example, in a preferred implementation the fixed division points areset at or slightly less than one-quarter of the CLK period. It will berecognized that it is easier to obtain frequency lock with a relativelylarger sized (wider) part OP1 (for example, at or more than one-half ofthe CLK period). However, the larger (wider) the part OP1, the moresusceptible the system is to jitter. Thus, it would be preferred, toaddress issues with jitter, to have a smaller (narrower) part OP1.

Reference is now made to FIG. 13 which illustrates an alternate circuitfor generating the signals CLKM, CLK and CLKP. In the alternate circuit,the first delay 380′ and second delay 382′ are capable of producingvariable delays. The first delay 380′ includes a plurality of firstfixed delays 380(1)-380(N) connected in series. A first plurality oftaps TM(1)-TM(N) are provided in the series connected first fixed delays380(1)-380(N). The second delay likewise includes a plurality of secondfixed delays 382(1)-382(N) connected in series. A second plurality oftaps TP(1)-TP(N) are provided in the series connected second fixeddelays 382(1)-382(N). A first switching circuit 381 is coupled to selectone of the first plurality of taps TM(1)-TM(N) and output the signalCLKM. A second switching circuit 383 is coupled to select one of thesecond plurality of taps TP(1)-TP(N) and output the signal CLKP. Thesignal CLK is taken at the series connection point 385 between the firstdelay 380′ and second delay 382′.

The size of the part OP1 is selected through operation of the firstswitching circuit 381 and second switching circuit 383. For example,relatively wider parts OP1 are provided when the first switching circuit381 and second switching circuit 383 choose lower numbered taps TP andTM (i.e., taps closer to the series connection point 385 between thefirst delay 380′ and second delay 382′ where signal CLK is obtained).Conversely, relatively narrower parts OP1 are provided when the firstswitching circuit 381 and second switching circuit 383 choose highernumbered taps TP and TM (i.e., taps farther away from the seriesconnection point 385 between the first delay 380′ and second delay 382′where signal CLK is obtained).

To perform a jitter optimization operation, the processes for frequencycontrol and phase control can start with a selection the first switchingcircuit 381 and second switching circuit 383 of a relatively wider partOP1. If no lock is obtained, the selected width of the part OP1 ismaintained. However, if lock is obtained at this selection, then a newselection is made by the first switching circuit 381 and secondswitching circuit 383 which slightly narrows the part OP1, and theprocess attempts to obtain lock at this slightly narrower part OP1. Theprocess is repeated with a narrowing of the part OP1 with eachconsecutive successful lock. In this way, a relatively wider part OP1may be used in connection with initial frequency and phase setting,while a relatively narrower part OP1 is used in a low jitter fine tuningprocess, specifically with respect to phase control.

While this detailed description has set forth some embodiments of thepresent invention, the appended claims cover other embodiments of thepresent application which differ from the described embodimentsaccording to various modifications and improvements. Other applicationsand configurations may be apparent to the person skilled in the art.

1. A clock recovery circuit, comprising: a control circuit adapted toreceive a first clock signal at a first frequency, a frequency divisionsignal specifying a divisor number, and a second clock signal at asecond frequency, wherein the second frequency is higher than the firstfrequency, the control circuit comprising: a phase control block adaptedto define non-overlapping portions of a pulse of the second clockincluding a center portion, left portion and right portion and determinewhether an edge of the first clock is located within the center portion;and a frequency control block responsive to the determination that theedge of the first clock is located within the center portion and adaptedto compare a number of periods of the second clock signal which occurwithin one or more periods of the first clock signal to a number derivedfrom the divisor number and generate a frequency selection signal inresponse thereto; and a controlled oscillator circuit adapted togenerate the second clock signal at the second frequency, wherein thesecond frequency is specified by the frequency selection signal.
 2. Thecircuit of claim 1, wherein the phase control block is further adaptedto determine whether an edge of the first clock is located within theleft portion and in response thereto generate a phase adjustment signal;and wherein the second frequency of the second clock signal generated bythe controlled oscillator circuit is specified by both the frequencyselection signal and the phase adjustment signal, the phase adjustmentsignal causing the edge of the first clock to shift towards the centerportion.
 3. The circuit of claim 1, wherein the phase control block isfurther adapted to determine whether an edge of the first clock islocated within the right portion and in response thereto generate aphase adjustment signal; and wherein the second frequency of the secondclock signal generated by the controlled oscillator circuit is specifiedby both the frequency selection signal and the phase adjustment signal,the phase adjustment signal causing the edge of the first clock to shifttowards the center portion.
 4. The circuit of claim 1, wherein the phasecontrol block is further adapted to determine whether an edge of thefirst clock is located within the left portion and in response theretogenerate a first phase adjustment signal; wherein the phase controlblock is further adapted to determine whether an edge of the first clockis located within the right portion and in response thereto generate asecond phase adjustment signal; and wherein the second frequency of thesecond clock signal generated by the controlled oscillator circuit isspecified by the frequency selection signal, the first phase adjustmentsignal and the second phase adjustment signal, the first and secondphase adjustment signals causing the edge of the first clock to shifttowards the center portion.
 5. The circuit of claim 1, wherein thecontrol circuit is adapted to operate in a calibration mode and a lockmode, wherein the calibration mode implements interleaved actuation ofthe phase control block and frequency control block, and wherein thelock mode implements actuation only of the phase control block.
 6. Thecircuit of claim 1, wherein the control circuit is adapted to operate ina fast calibration mode, a fine calibration mode and a lock mode,wherein both the fast and fine calibration modes each implementinterleaved actuation of the phase control block and frequency controlblock, and wherein the lock mode implements actuation only of the phasecontrol block.
 7. The circuit of claim 6, wherein actuation of thefrequency control block in the fast calibration mode compares the numberof periods of the second clock signal which occur within only one periodof the first clock signal to the divisor number itself and generates thefrequency selection signal in response thereto.
 8. The circuit of claim7, wherein actuation of the frequency control block in the finecalibration mode compares the number of periods of the second clocksignal which occur within a plurality of N periods of the first clocksignal to the number equal to N times the divisor number and generatesthe frequency selection signal in response thereto.
 9. The circuit ofclaim 1, wherein the frequency selection signal is generated by thefrequency control block by execution of a binary search algorithm. 10.The circuit of claim 1, wherein the frequency selection signal isgenerated by the frequency control block by increment/decrementalgorithm.
 11. The circuit of claim 10, wherein each increment ordecrement is a single step.
 12. The circuit of claim 1, wherein a sizeof each of the center portion, left portion and right portion is fixed.13. The circuit of claim 1, wherein a size of each of the centerportion, left portion and right portion is variable.
 14. The circuit ofclaim 13, wherein the size of the center portion is wider duringfrequency selection and narrower during phase selection.
 15. The circuitof claim 1, wherein the frequency control block is further adapted togenerate the frequency selection signal having a fractional value.
 16. Amethod, comprising: receiving a first clock signal at a first frequency;receiving a frequency division signal specifying a divisor number;receiving a second clock signal at a second frequency, wherein thesecond frequency is higher than the first frequency; definingnon-overlapping portions of a pulse of the second clock including acenter portion, left portion and right portion; determining whether anedge of the first clock is located within the center portion; when theedge of the first clock is located within the center portion, comparinga number of periods of the second clock signal which occur within one ormore periods of the first clock signal to a number derived from thedivisor number; generating a frequency selection signal in response tosaid comparison; and generating the second clock signal at the secondfrequency, wherein the second frequency is specified by the frequencyselection signal.
 17. The method of claim 16, further comprising:determining whether an edge of the first clock is located within theleft portion; and generating a phase adjustment signal in response tosaid determination; wherein generating the second clock signal comprisesgenerating the second clock signal at the second frequency, wherein thesecond frequency is specified by both the frequency selection signal andthe phase adjustment signal, the phase adjustment signal causing theedge of the first clock to shift towards the center portion.
 18. Themethod of claim 16, further comprising: determining whether an edge ofthe first clock is located within the right portion; and generating aphase adjustment signal in response to said determination; whereingenerating the second clock signal comprises generating the second clocksignal at the second frequency, wherein the second frequency isspecified by both the frequency selection signal and the phaseadjustment signal, the phase adjustment signal causing the edge of thefirst clock to shift towards the center portion.
 19. The method of claim16, first determining whether an edge of the first clock is locatedwithin the left portion; and generating a first phase adjustment signalin response to said first determination; second determining whether anedge of the first clock is located within the right portion; andgenerating a phase adjustment signal in response to said seconddetermination; wherein generating the second clock signal comprisesgenerating the second clock signal at the second frequency, wherein thesecond frequency is specified by the frequency selection signal, thefirst phase adjustment signal and the second phase adjustment signal,the first and second phase adjustment signals causing the edge of thefirst clock to shift towards the center portion.
 20. The method of claim16, further comprising: operating in a calibration mode and a lock mode;wherein the calibration mode implements an interleaved phase controloperation and frequency control operation, and wherein the lock modeimplements only the phase control operation.
 21. The method of claim 16,further comprising: operating in a fast calibration mode, a finecalibration mode and a lock mode; wherein both the fast and finecalibration modes implement an interleaved phase control operation andfrequency control operation, and wherein the lock mode implements onlythe phase control operation.
 22. The method of claim 21, whereincomparing the number of periods of the second clock signal which occurwithin one or more periods of the first clock signal to the numberderived from the divisor number comprises, when in the fast calibrationmode, comparing the number of periods of the second clock signal whichoccur within only one period of the first clock signal to the divisornumber itself.
 23. The method of claim 22, wherein comparing the numberof periods of the second clock signal which occur within one or moreperiods of the first clock signal to the number derived from the divisornumber comprises, when in the fine calibration mode, comparing thenumber of periods of the second clock signal which occur within aplurality of N periods of the first clock signal to the number equal toN times the divisor number and generates the frequency selection signalin response thereto.
 24. The method of claim 16, wherein generating afrequency selection signal comprises executing a binary searchalgorithm.
 25. The method of claim 16, wherein generating a frequencyselection signal comprises executing an increment/decrement algorithm.26. The method of claim 25, wherein each increment or decrement is asingle step.
 27. The method of claim 16, further comprising fixing asize of each of the center portion, left portion and right portion. 28.The method of claim 16, further comprising varying a size of each of thecenter portion, left portion and right portion.
 29. The method of claim28, wherein the size of the center portion is wider during frequencyselection and narrower during phase selection.
 30. The method of claim15, wherein the frequency selection signal has a fractional value.